1. Field of the Invention:
The present invention generally relates to a heating circuit for a filament of an X-ray tube, and more particularly, to the filament heating circuit utilizing a voltage resonance type DC-to-DC converter.
2. Description of the Prior Art:
In an X-ray diagnostic apparatus, such as an X-ray computerized tomographic (CT) apparatus or a digital fluoroscopic apparatus, the most important aspect is to realize a stable X-ray generation. It is therefore necessary to stabilize the application of high voltage to an anode of an X-ray tube, and also to heat (power supply) a filament (cathode) of the X-ray tube.
Various types of filament heating circuits have been proposed. For instance, according to a first conventional heating circuit, a ferroresonant stabilizer is used in combination with series-connected resistors, whereby the voltage of the primary circuit of a transformer is controlled to be stable by utilizing the voltage drop across the resistors. This conventional heating circuit has the following drawbacks. That is, the response speed of the filament heating is considerably low because it is restricted by the frequency of the power supply, i.e., 50 HZ or 60 HZ. Secondly, due to the inherent matter of the circuit arrangement, a stable heating cannot be substantially realized when the equivalent resistance of the filament changes during operations. This resistance includes not only the filament resistance per se, but also an internal resistance of the high voltage cables through which high voltage is applied to the X-ray tube. Moreover, if a rectifier diode bridge circuit and a smoothing capacitor are employed in the secondary winding circuit of the transformer, an AC current flowing through the primary winding is disturbed to a great extent during the discharge period of the capacitor. This causes the primary voltage of the transformer to be astable because of the characteristic of the ferroresonant stabilizer.
The switching regulator type filament heating circuit has been also proposed. According to this heating circuit, a limitation exists in the switching frequency, e.g., 100 to 200 HZ. If a higher switching frequency is selected for such a heating circuit, a greater loss of the power transmission in the transformer may occur. This is caused by a leakage inductance between the primary and secondary windings of the transformer.